I had no idea Rubbermaid made tanks. If rays will take higher temps then you should be good to go. However it would be a good idea to keep your eyes peeled for that fridge. In a heat wave it could mean the difference in having your rays or having some new ones. At what rays cost, it might even be a good idea to buy a new fridge just for the reliability factor.
A good carpenter never blames his tools...(say it was the internet)
You can try measuring the ground temp at say 6-8ft, if it's reasonably cool thro your coils in the bottom pipe them into your system an repack the ground. This will help stabilize the temp. You can use copper, s.s. or poly pipe in either idea.